Micromirror and post arrangements on substrates

ABSTRACT

A micromirror of a micromirror array of a spatial light modulator used in display systems comprises a mirror plate attached to a hinge that is supported by two posts formed on a substrate. Also the mirror plate is operable to rotate along a rotation axis that is parallel to but offset from a diagonal of the mirror plate when viewed from the top. An imaginary line connecting the two posts is not parallel to either diagonal of the mirror plate.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This US patent application is a continuation of co-pending U.S. patentapplication Ser. No. 10/698,563 filed Oct. 30, 2003 to Huibers, thesubject matter of which is incorporated herein by reference in entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is related generally to the art ofmicroelectromechanical systems, and, more particularly, to micromirrorarray devices comprising a plurality of micromirrors for use in displayand other systems.

BACKGROUND OF THE INVENTION

Spatial light modulators (SLMs) are transducers that modulate anincident beam of light in a spatial pattern in response to an optical orelectrical input. The incident light beam may be modulated in phase,intensity, polarization, or direction. This modulation may beaccomplished through the use of a variety of materials exhibitingmagneto-optic, electro-optic, elastic or other properties. SLMs havemany applications, including optical information processing, displaysystems, and electrostatic printing.

Therefore, what is needed is a spatial light modulator that has a highresolution, a high fill factor and a high contrast ratio. What isfurther needed is a spatial light modulator that does not requirepolarized light, hence is optically efficient and mechanically robust.

SUMMARY OF THE INVENTION

The present invention provides a spatial light modulator that has a highresolution, a high fill factor and a high contrast ratio.

In an embodiment of the invention, a spatial light modulator isdisclosed. The spatial light modulator comprises: an array ofmicromirrors on a substrate, each micromirror having four predominantsides and being held on the substrate by a plurality of posts, whereinthe four predominant sides define two diagonals, and wherein a linebetween any two of the plurality of posts is not coincident with eitherof the two diagonals.

In another embodiment of the invention, another spatial light modulatoris disclosed. The spatial light modulator comprises: a rectangular arrayof micromirrors on a substrate, each micromirror having four predominantsides, wherein each side is neither parallel nor perpendicular to theedges of the rectangular array.

In yet another embodiment of the invention, a projection system isdisclosed. The projection system comprises: a light source; a spatiallight modulator that further comprises an array of micromirrors formedon a rectangular substrate, each micromirror having a rectangular mirrorplate held on the substrate; a condensing lens for directing light fromthe light source onto the spatial light modulator, wherein light fromthe light source is directed onto the micromirror array at an incidentangle of from 50° to 70° degrees relative to the substrate plane and atan angle of from 50° to 65° degrees relative to a side of the substratewhen viewed from the top; and a projection lens for directing light fromthe spatial light modulator onto a display target.

In yet another embodiment of the invention, a spatial light modulator isdisclosed. The spatial light modulator comprises: an array ofmicromirrors on a substrate, each micromirror held on the substrate by aplurality of posts, each micromirror having four predominant sides andwherein the four predominant sides define two diagonals, wherein a linedrawn between any two of the posts is not coincident with either of thetwo diagonals.

In yet another embodiment of the invention, a spatial light modulator isdisclosed. The spatial light modulator comprises: an array ofmicromirrors, wherein imaginary lines connecting centers of eachmicromirror in the array form an imaginary grid of rows and columnsorthogonal to each other, and wherein the edges of the micromirrors arenot parallel to either the rows or columns in the imaginary grid.

In yet another embodiment of the invention, a spatial light modulator isdisclosed. The spatial light modulator comprises: an array ofmicromirrors on a substrate, each micromirror held on the substrate by ahinge connected to the micromirror and to two posts on the substrate,each micromirror having an axis of rotation, wherein a line drawnbetween the two posts is not parallel with the axis of rotation.

BRIEF DESCRIPTION OF DRAWINGS

While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a diagram schematically illustrates an exemplary displaysystem employing a spatial light modulator having an array ofmicromirrors;

FIG. 2 is a diagram schematically illustrates another exemplary displaysystem employing three spatial light modulators, each having an array ofmicromirrors;

FIG. 3 is a cross-sectional view of the spatial light modulator of FIG.1 and FIG. 2;

FIG. 4A is a top view of an array of micromirrors of the presentinvention;

FIG. 4B illustrates sets of imaginary lines connecting the centers ofthe micromirrors of the micromirror array of FIG. 4A;

FIG. 5 a through FIG. 5 e illustrate a micromirror in accordance with anembodiment of the present invention, wherein FIG. 5 a is a perspectiveview of the micromirror; FIG. 5 b is a top view of the micromirror inFIG. 5 a; FIG. 5 c is a cross-sectional view of the micromere in FIG. 5a; FIG. 5 d is a cross-sectional view of the micromirror in FIG. 5 a;and FIG. 5 e is another cross-sectional view of the micromirror in FIG.5 a; and

FIG. 6 a and FIG. 6 b are cross-sectional views of the micromirror inFIG. 5 a during an exemplary fabrication process in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Processes for micro-fabricating a microelectromechanical system (MEMS)device such as a micromirror and micromirror array are disclosed in U.S.Pat. Nos. 5,835,256 and 6,046,840 both to Huibers, the subject matter ofeach being incorporated herein by reference. A similar process forforming MEMS movable elements (e.g. micromirrors) on a wafer substrate(e.g. a light transmissive substrate or a substrate comprising CMOS orother circuitry) is illustrated in the present application.

U.S. patent applications Ser. No. 09/910,537 filed Jul. 20, 2001, and60/300,533 filed Jun. 22, 2001 both to Reid, and U.S. patentapplications Ser. No. 10/176,478 filed Jun. 21, 2002 to Reid, and U.S.patent application Ser. No. 10,365,951 filed Feb. 12, 2003 containexamples of materials that may be used for the various components of thecurrent invention. U.S. patent applications Ser. No. 09/564,069 toRichards, filed May 3, 2000, Ser. No. 10/340,162 to Richards, filed Jan.10, 2003, Ser. No. 10/407,061 to Richards, filed Apr. 2, 2003, and Ser.No. 10/607,687 to Richards, filed Jun. 27, 2003 contain examples ofoperation methods that can be used to operate the micromirror and themicromirror array, especially in display applications. And U.S. patentapplications Ser. No. 10/366,296 to Patel, filed Feb. 12, 2003, Ser. No.10/366,297 to Patel, filed Feb. 12, 2003 and Ser. No. 10/613,379 toPatel filed Jul. 3, 2003 contain exemplary configurations of themicromirror and the micromirror array that can be used in theembodiments of the present invention. These patent applications areincorporated herein by reference.

The present invention discloses a spatial light modulator that comprisesan array of micromirrors for spatially modulating light. The spatiallight modulator and the micromirror array within the spatial lightmodulator have a variety of applications, such as digital displaysystems, maskless lithography, atomic spectroscopy, maskless fabricationof DNA microarrays, signal processing, and microscopy.

Turning to the drawings, FIG. 1 is a diagram schematically illustratingan exemplary display system in which the various embodiments of theinvention can be implemented. Referring to FIG. 1, in one basicconfiguration, the display system comprises light source 102, opticaldevices (e.g. light pipe 104, collection optics 106 and projectionoptics 108), and spatial light modulator 110 that further comprises anarray of micromirrors. Also shown is a display target 112 onto which animage is directed. Light source 102 (e.g. an arc lamp) directs lightthrough the light integrator/pipe 104 and collection optics 106 and ontospatial light modulator 110. The micromirrors of the spatial lightmodulator 110 are selectively actuated by a controller (e.g. asdisclosed in U.S. Pat. No. 6,388,661 issued May 14, 2002 incorporatedherein by reference) so as to reflect—when in their “ON” position—theincident light into projection optics 108, resulting in an image ondisplay target 112 (screen, a viewer's eyes, a photosensitive material,etc.). Exemplary operation methods are disclosed in U.S. Pat. No.6,388,661, and U.S. patent application Ser. No. 10/340,162, filed onJan. 10, 2003, both to Richards, the subject matter of each beingincorporated herein by reference. Generally, more complex opticalsystems are often used, especially in displaying applications for colorimages, such as the display system in FIG. 2.

Referring to FIG. 2, another exemplary display system employing threespatial light modulators, each comprising an array of micromirrors andbeing designated for respectively modulating the multi-color (e.g. threecolor such as red, green and blue) light beams, is presented therein.The display system employs a dichroic prism assembly 204 for splittingincident light into three primary color light beams. Dichroic prismassembly comprises TIR 176 a, 176 c, 176 d, 176 e and 176 f.Totally-internally-reflection (TIR) surfaces, i.e. TIR surfaces 205 a,205 b and 205 c, are defined at the prism surfaces that face air gaps.The surfaces 198 a and 198 b of prisms 176 c and 176 e are coated withdichroic films, yielding dichroic surfaces. In particular, dichroicsurface 198 a reflects green light and transmits other light. Dichroicsurface 198 b reflects red light and transmits other light. The threespatial light modulators, 182, 184 and 186, each having a micromirrorarray device, are arranged around the prism assembly.

In operation, incident white light 174 from light source 102 enters intoTIR 176 a and is directed towards spatial light modulator 186, which isdesignated for modulating the blue light component of the incident whitelight. At the dichroic surface 198 a, the green light component of thetotally internally reflected light from TIR surface 205 a is separatedtherefrom and reflected towards spatial light modulator 182, which isdesignated for modulating green light. As seen, the separated greenlight may experience TIR by TIR surface 205 b in order to illuminatespatial light modulator 182 at a desired angle. This can be accomplishedby arranging the incident angle of the separated green light onto TIRsurface 205 b larger than the critical TIR angle of TIR surface 205 b.The rest of the light components, other than the green light, of thereflected light from the TIR surface 205 a pass through dichroic surface198 a and are reflected at dichroic surface 198 b. Because dichroicsurface 198 b is designated for reflecting red light component, the redlight component of the incident light onto dichroic surface 198 b isthus separated and reflected onto spatial light modulator 184, which isdesignated for modulating red light. Finally, the blue component of thewhite incident light (white light 174) reaches spatial light modulator186 and is modulated thereby. By collaborating operations of the threespatial light modulators, red, green and blue lights can be properlymodulated. The modulated red, green and blue lights are recollected anddelivered onto display target 112 through optic elements, such asprojection lens 202, if necessary.

The spatial light modulator, in general, comprises an array of thousandsor millions of micromirrors. FIG. 3 illustrates a cross-sectional viewof a portion of an exemplary spatial light modulator of FIG. 1 and FIG.2. For simplicity and demonstration purposes, micro-mirrors andelectrodes are shown in diagrammatical sketches and only ninemicromirrors and electrodes are illustrated therein for convenience.Referring to FIG. 3, micromirror array device 110 comprises an array ofmicromirrors (e.g. micromirror 122) formed on substrate 120, which canbe a glass or quartz substrate transmissive to visible light in thisexample. A typical size of the micro-mirror is a few micrometers orless. The glass or quartz substrate may have optical films, such as ananti-reflection film coated thereon. The micromirrors spatially modulatethe incident light by selectively reflecting the incident light onto oraway from a projection lens (e.g. projection lens 108 in FIG. 1 a) forproducing images or videos on a display target (e.g. display target 112in FIG. 1 a). The selective reflection of the incident light by themicromirrors is accomplished through an array of electrodes (e.g.electrode 126) and circuitry (not shown). Specifically, each micromirrorcomprises a mirror plate, which is associated with an electrode. Anelectrostatic field can thus be established between the mirror plate andthe associated electrode. In response to the established electrostaticfield, the mirror plate rotates to either an ON state or an OFF state.In the ON state, the mirror plate reflects the incident light into theprojection lens, and in the OFF state, the mirror plate reflects theincident light away from the projections lens. In this particularexample, the array of electrodes and circuitry is formed on substrate124, which is preferably a die from a semiconductor wafer.Alternatively, micromirrors can be formed on the same substrate as thaton which are formed the electrodes and circuitry—though this embodimentis not shown herein. As will be seen in later drawings, the hingearrangement and rotation illustrated in FIG. 3 is simplified.

Referring to FIG. 4A, a top view of the spatial light modulator in FIG.3 is illustrated therein. Spatial light modulator 110 comprises an arrayof micromirrors (e.g. micromirror 216) that are formed on lighttransparent substrate 210. Though nine micromirrors are illustrated inFIG. 4A, because the number of micromirrors of the micromirror arraydetermines the resolution of the display system, in real displayapplications, the micromirror array has thousands or millions ofmicromirrors. In the present invention, the micromirror array preferablyhas 1024×768 micromirrors, 1280×720, 1400×1050, 1600×1200, 1920×1080, orother desired resolution.

In accordance with an embodiment of the invention, each micromirror(e.g. micromirror 216 or micromirror 217) has four predominant sides andis held on the substrate by a plurality of posts (e.g. posts 218, whichare better illustrated in FIG. 6 a). A four sided micromirror aids in“tiling” the mirrors together so as to minimize gaps and othernon-reflective areas. The four predominant sides define two diagonals,such as the diagonals plotted in solid lines in micromirror 217. A linebetween the centers of any two of the plurality of posts, such as a linebetween the centers of posts 219 a and 219 b in micromirror 217 is notcoincident with either of the two diagonals. This is due to the factthat the posts that hingedly support the micromirror plates are not atthe direct corners of the micromirrors. By having the micromirrors“skewed” such that the posts are not at the direct corners of the mirrorplats, both the mirror plates and posts can be “tiled” with maximumeffect (minimizing non-reflecting areas).

In another way of describing this arrangement of the invention, thecenters of the micromirrors in each row (the micromirrors disposed alongthe X-direction) are connected with an imaginary line, and the imaginarylines for all rows form a set of parallel imaginary lines along theX-direction, as shown in FIG. 4B. The centers of the micromirrors ineach column (the micromirrors disposed along the Y-direction) areconnected with another imaginary line, and all such imaginary lines formsets of parallel imaginary lines along the Y-direction. The imaginarylines along the X-direction are orthogonal to the imaginary lines alongthe Y-direction, and all imaginary lines form an orthogonal grid. Eachone of the four predominant edges of each micromirror, however, is notparallel to an imaginary line of the grid.

In accordance with another embodiment of the invention, spatial lightmodulator 110, substrate 210 and the micromirror array formed on thesubstrate are rectangular in shape as illustrated in the figure. By“rectangular micromirror array”, it is meant that a line (e.g. dottedline 213 a in FIG. 4A) connecting all equivalent points (e.g. points a,b and c in FIG. 4A) of the micromirrors (e.g. micromirrors 215 a, 215 band 215 c) along the X direction (e.g. the X-direction in the X-YCartesian coordinate system illustrated in FIG. 4A) is perpendicular orsubstantially perpendicular to a line (e.g. dotted line 213 b)connecting all equivalent points (e.g. points d, e and f) of themicromirrors (e.g. micromirrors 215 c, 215 d and 215 e) along the Ydirection of the X-Y Cartesian coordinate system. Each micromirror hasfour predominant sides, wherein each side is neither parallel norperpendicular to the edges of the rectangular micromirror array. Forexample, edge 216 a of micromirror 216 has an angle θ with the edge(e.g. the edge represented by dotted lines 213 b) of the micromirrorarray, wherein the angle θ is neither 0° nor (n·90)° (n is an integernumber) degrees. In the present invention, the angle θ can be an anglefrom 2° to 30°, though more likely within the range of from 5° to 25°degrees, (e.g. from 10° to 20° degrees, or around 15° degrees).

Angle θ relates to the ratio of the width of the mirror plate to thewidth of the post. For example, assuming both of the post and mirrorplate are square, angle θ can then be written as: ctgθ=(m/p−1), whereinm is the width of the mirror plate and p is the width of the post.Corresponding to the preferred range of angle θ from 2° to 30°, theratio of m/p ranges from 0.7 to 27.6.

In accordance with the invention, spatial light modulator 110 having themicromirror array can be employed in a display system (e.g. displaysystem in FIG. 1 or FIG. 2) for modulating the light beam from a lightsource (e.g. light source 102 in FIG. 1 or FIG. 2) and condensingoptics, such as condensing lens 106 in FIG. 1. The light beam isdirected onto the micromirror array at a certain angle, which is betterillustrated in FIG. 3. Referring back to FIG. 3, incident light beam 220has an angle φ relative to substrate 210. The incident angle φ ispreferably from 50° to 70° degrees. The incident light also has an anglewith the edges of the micromirror array, which is better illustrated inFIG. 4A. As can be seen in FIG. 4A, incident light angle 220 has anangle ω relative to edge 213 a of the micromirror array. It is preferredthat angle ω is from 50° to 65° degrees.

In accordance with another aspect of the invention, each micromirror hasan axis of rotation, wherein a line drawn between the two posts is notparallel with the axis of rotation. This is illustrated in FIG. 5 a.Referring to FIG. 5 a, a perspective view of a portion of an exemplarymicromirror of the micromirror array in FIG. 4A is illustrated therein.As seen, hinge support 219 is formed on substrate 210, which can be alight transmissive substrate such as a glass substrate transmissive tovisible light. The hinge support connected to the substrate can includetwo posts 218. Hinge 227 is affixed to the hinge support. Mirror plate232 is attached to hinge 227 via hinge connector 228. In this particularexample, the hinge contact 228 is disposed at a location not at thecenter of the mirror plate. This configuration facilitates the mirrorplate rotating along a rotational axis that is parallel to but offsetfrom the diagonal of the mirror plate when viewed from the top ofsubstrate 210. By “parallel to but offset from the diagonal”, it ismeant that the axis of rotation can be exactly parallel to orsubstantially parallel to (±10° degrees) the diagonal of themicromirror. Such a rotation axis can be achieved by attaching the hingestructure to the mirror plate at a point not along the mirror platediagonal 211. The point of attachment can be at least 0.5 um, at least 1um, or at least 2 um away from the diagonal 211. In one embodiment, thepoint of attachment is from 1/40 to ⅓ the length of the diagonal awayfrom diagonal 211, or from 1/20 to ¼ if desired—although any desireddistance away from the diagonal is possible if so desired in the presentinvention. In the present invention, the micromirror preferably has asubstantially four-sided shape. Whether the micromirror is a rectangle,square, rhombus or trapezoid, even if the corners are rounded or“clipped” or if an aperture or protrusion is located on one or more ofthe sides of the micromirror, it is still possible to conceptuallyconnect the four major sides of the micromirror shape and take adiagonal across the middle of the micromirror. In this way, a centerdiagonal can be defined even if the micromirror plate is substantiallybut not perfectly a rhombus, trapezoid, rectangle, square, etc. However,the rotation axis of the micromirror plate is not along the centerdiagonal but is along direction 213 that is parallel to but offset fromdiagonal 211 when viewed from the top in FIG. 3A. This type of designbenefits the performance of the micromirror device in a number of ways.One advantage of this asymmetric offset arrangement is that themicromirror plate can rotate at a larger angle than the rotation anglethat can be achieved in a symmetrical arrangement (with a mirrorplate—substrate gap being the same). The length of the diagonal of themirror plate is preferably 25 microns or less.

In addition to the mirror plate, the hinge and the hinge contact,additional features are provided for the micromirror according to thepresent invention. For example, extension-plate 234 is constructed onmirror plate 232 for enhancing electrostatic coupling of the mirrorplate with the adjacent electrode (not shown—see FIG. 3) that isprovided for driving the mirror plate to rotate relative to thesubstrate. As can be seen in the figure, the extension-plate isconnected to the mirror plate via an extension-plate post 236 and on theopposite side of the mirror plate to substrate 210 to which the hingesupport and the hinge are connected. The extension-plate post 236 isdisposed at a location not at the center of the mirror plate. Moreover,the location of the extension-plate post 236 at the mirror plate is notalong a line connecting the two posts 218. The extension-plate isconnected to the substrate via the hinge connect, the hinge, the hingesupport and the two posts. The relative position of the mirror plate,the hinge and the extension-plate is better illustrated in FIGS. 5 b and5 d.

Referring to FIG. 5 b, a top view of the micromirror in FIG. 5A isillustrated therein. The dash line E shows a diagonal of the mirrorplate. The dotted line D represents an imaginary line connecting the twoposts of the micromirror device. And the dash-and-dotted line Fillustrates the location of the hinge to which the mirror plate isattached. It can be seen that, the hinge represented by line F isparallel to but offset from the diagonal represented by line E. And thediagonal represented by line E crosses the line D connecting the twoposts.

The mirror plate and the hinge are not on the same plane, which isbetter illustrated in FIG. 5 c. FIG. 5 c illustrates a cross-sectionalview of the micromirror in FIG. 5 a along line CC. Mirror plate 232 isformed on substrate 210. Hinge 227 is formed on the mirror plate andconnected to the mirror plate via hinge post 228 a. Referring to FIG. 5d, a cross-sectional view of the micromirror along line AA in FIG. 5 ais illustrated therein. Mirror plate 232 is above substrate 210. Hingecontact 228 connects hinge 227 (in FIG. 5 a) to the mirror plate.Extension-plate 234 is constructed on the mirror plate. Referring toFIG. 5 e, another cross-sectional view of the micromirror along line BBin FIG. 5 a is illustrated therein. As seen, two posts 218 are formed onthe substrate. Mirror plate is held on the substrate and extension-plate234 is constructed on the mirror plate. Referring back to FIG. 5 a, theextension-plate as illustrated is within the micromirror. However, thisis not an absolute requirement. Rather, the extension-plate can beextended beyond the micromirror. In particular, the extension-plate of amicromirror can be extended beyond the micromirror having the mirrorplate to which said extension-plate is connected into adjacentmicromirrors in a micromirror array. This design further enhancescoupling of the micromirror to the electrostatic fields, because thecoupling is increased with the area of the extension-plate increased.

In an embodiment of the invention, the extension plate is metallic, andis electrically connected to the mirror plate. In operation, the mirrorextension plate is held at the same voltage as the mirror plate.However, because the extension plate is closer to the electrode than themirror plate, electrostatic force exerted to the extension plate islarger than the force to the mirror plate. That is, compared to therequired voltage difference between the electrode and the mirror plateto rotate the mirror plate to a desired angle, a smaller voltagedifference between the extension plate and the electrode will be enoughto rotate the mirror plate to the same desired angle. In the embodimentof the invention, the extension plate has the same distance from themirror plate as the distance between hinge 227 and the mirror plate.This type of arrangement simplifies the fabrication of the micromirrordevice. In another embodiment of the invention, the extension plate hasa different distance from the mirror plate than the hinge. For example,the distance between the extension plate and the mirror plate is largerthan the distance between the hinge and the mirror plate. In thissituation, the required voltage difference can be even smaller toachieve the desired rotation angle than the necessary voltage differenceby the micromirror to achieve the same desired angle, wherein the hingeand the extension plate have the same distance from the mirror plate.

Alternatively, the extension-plate can be a dielectric plate having adielectric constant larger than 1. In operation, when voltages areapplied to the electrode and the mirror plate, resulting in a voltagedifference between the electrode and the mirror plate, the electricforce exerted to the mirror plate is larger than the electric forceexerted to the mirror plate resulted from the same voltage differenceestablished between the mirror plate and the electrode withoutdielectric plate in between. That is, compared to the required voltagedifference between the electrode and the mirror plate to rotate themirror plate to a desired angle, a smaller voltage difference isnecessary to rotate the mirror plate to the same desired angle. Similarto the embodiment wherein the extension plate is metallic, thedielectric extension plate can be spaced from the mirror plate witheither the same or a different distance as that between the mirror plateand the hinge.

In addition to the extension-plate, the micromirror may include otheradditional features. For example, stops 226 a and 226 b can be part ofthe hinge support for stopping the rotation of the mirror plate to theON state, and it can thus be used to define a uniform ON state angle forthe micromirrors of the micromirror array device. Stop 230 may also be apart of the hinge support. This stop is better illustrated in thecross-sectional view of FIG. 5 b. Referring back to FIG. 5 b, stop 230is formed on the hinge support and is extended towards the mirror platesuch that the clockwise rotation of the mirror plate can be stopped bystop 230 when mirror plate achieves a certain rotation angle. The valueof the angle (OFF state angle) is determined by the location (e.g. thedistance from hinge contact 228) of stop 230 and the height of stop 230.This stop can be used to define a uniform OFF state for the mirror plateof the micromirror and the micromirror array. In order to drive themirror plate to rotate relative substrate to the OFF state, anotherelectrode (not shown) is provided.

When the desired OFF state angle of the micromirror is non-zero, asecond electrode other than the first electrode that drives the mirrorplate to rotate to the ON state angle can be provided for driving themirror plate to rotate to the OFF state angle. Or, simply a voltage biascan be applied between the mirror plate and substrate 210 (e.g. aconductive coating thereon). In order for the mirror plate to rotate toa second OFF state rotation direction that is opposite to the firstrotation direction, a first electric field is established between themirror plate/extension plate and the electrode on the opposing substratefor driving the mirror plate to rotate to the ON state angle. And whenthis field for the ON state is removed, the bias on the substrate 210will naturally pull back the mirror plate to a non-flat OFF state.

More particularly, the bias on substrate 210 is preferably anelectrically conductive film deposited on the surface of the substrate210 and the electrically conductive film is preferably transmissive tovisible light. In operation, an electric potential is applied to thefilm, and the electric potential can be maintained during the entireoperation of the micromirror. In this situation, the electric potentialpulls the mirror plate to the OFF state when the ON state electric fieldbetween the extension-plate and the first electrode is not present.Otherwise, the electrostatic force between the ON state electrode andthe extension-plate overcomes any bias from the substrate 210, such thatthe mirror plate rotates to the ON state from the OFF state.

Instead of providing stop 230 for stopping the rotation of the mirrorplate when the OFF angle is achieved, other stopping mechanisms may alsobe provided for achieving the same purpose, such as those disclosed inU.S. patent application Ser. No. 10,437,776 to Patel, filed May 13,2003;

There is a variety of ways to construct the micromirror devicesdescribed above. Exemplary processes will be discussed in the followingwith references to FIGS. 6 a and 6 b. It should be appreciated by thoseordinary skills in the art that the exemplary processes are fordemonstration purpose only and should not be interpreted as limitations.

FIG. 6 a and FIG. 6 b are cross-sectional views of the micromirror inFIG. 5 a at different steps in an exemplary fabrication process for themicromirror, wherein the micromirror has an extension plate spaced fromthe mirror plate with the same distance as the hinge. FIG. 6 a is thecross-sectional view corresponds to the cross-sectional view of FIG. 5a, and FIG. 6 b is the cross-sectional view corresponds to thecross-sectional view of FIG. 5 b. Referring to FIG. 6 a, substrate 210is provided. First sacrificial layer 240 is deposited on the substratefollowed by the deposition of mirror plate layer 232. The substrate canbe a glass (e.g. 1737F, Eagle 2000), quartz, Pyrex™, or sapphire. Thesubstrate may also be a semiconductor substrate (e.g. silicon substrate)with one or more electrodes and/or control circuitry (e.g. CMOS typeDRAM) formed thereon. The first sacrificial layer may be any suitablematerial, such as amorphous silicon, or could alternatively be a polymeror polyimide, or even polysilicon, silicon nitride, silicon dioxide,etc. depending upon the choice of sacrificial materials, and the etchantselected. If the first sacrificial layer is amorphous silicon, it can bedeposited at 300-350° C. The thickness of the first sacrificial layercan be wide ranging depending upon the size of the micromirror deviceand desired maximum rotation angle of the mirror plate of themicromirror device, though a thickness of from 500 Å to 50,000 Å,preferably around 10,000 Å, is preferred. The first sacrificial layermay be deposited on the substrate using any suitable method, such asLPCVD or PECVD.

As an alternative feature of the embodiment, an anti-reflection layer(not shown) maybe deposited on the surface of the substrate. Theanti-reflection layer is deposited for reducing the reflection of theincident light from the surface of the substrate. Other opticalenhancing layers may also be deposited on either surface of the glasssubstrate as desired. In addition to the optical enhancing layers, anelectrical conducting layer can be deposited on a surface of thesubstrate. This electrical conducting layer can be used as an electrodefor driving the mirror plate to rotate, especially to an OFF state.Moreover, a light blocking area may be deposited on the glass substrateas set forth in U.S. patent applications Ser. No. 10,305,631 to Huibers,filed Nov. 26, 2002, the subject of each being incorporated herein byreference. The light area is deposited around the micromirrors in a formof frame, grid, strips or discontinuous segments such that lightscattered from the edges or other structures than the mirror plates ofthe micromirror can be suppressed if not removable.

After depositing the first sacrificial layer, mirror plate layer 232 fora micromirror plate is going to be deposited. Because the micromirrorplate is designated for reflecting incident light in the spectrum ofinterest (e.g. visible light spectrum), it is preferred that themicro-mirror plate layer comprises of one or more materials that exhibithigh reflectivity (preferably 90% or higher) to the incident light.Examples of such materials are Al, Ti, Ag, AlSi_(x)Cu_(y), AlTi_(x) orAlSi_(x). Of course, other suitable materials having high reflectivityto the incident light of interest may also be adopted for the mirrorplate.

Materials, such as Al, for the micromirror plate layer, however, candiffuse into and/or interact with the sacrificial layer, such as anamorphous silicon layer. The diffusion and interaction certainlydeteriorate the expected property of the micromirror plate and desiredperformance of the micromirror device. Specifically, the diffusion andinteraction may create defects on the micromirror device. In order toprevent such diffusion and interaction, a barrier layer that will beremoved with the sacrificial layer afterwards is deposited betweensacrificial layer 240 and mirror plate layer 136. Examples of suchbarrier layer and methods of using the barrier layer in fabricatingmicromirrors are disclosed in U.S. patent application Ser. No.10,402,789 to Patel, filed Mar. 28, 20023, the subject of which beingincorporated herein by reference. It is preferred that there issubstantially no interaction or diffusion between the barrier layer andthe micromirror plate, or substantially no interaction or diffusionbetween the barrier layer and the micromirror plate within thefabrication time period, such as 30 minutes, and under 400° C. (degree),which is the highest temperature during the fabrication time period. Itis further preferred that there is substantially no interaction ordiffusion between the barrier layer and the sacrificial layer, orsubstantially no interaction or diffusion between the barrier layer andthe sacrificial layer within the certain time period and under thecertain temperature. Exemplary barrier materials for the barrier layerare early transition metal elements (e.g. W, Ti, Ta and Mo), earlytransition metal alloys (e.g. WTi_(x), WMo_(x), and WTa_(x)), earlytransition metal compounds (e.g. WAl_(x)) including early transitionmetal silicides (e.g. WSi_(x), MoSi_(x), TiSi_(x), ZrSi_(x), CrSi_(x),TaSi_(x) and TiW_(x)Si_(y)). Other early transition metal silicide, suchas WN_(x) and CoSi_(x), are also applicable. The early transition metalsare those elements in the periodic table in columns 3 through 7, namelycolumns beginning with Sc, Ti, V, Cr, and Mn. Preferred are thoseelements in columns 4 through 6 (i.e. Ti, Zr, Hf, V, Nb, Ta, Cr, Mo andW). Of course, other suitable barrier materials that prevent thediffusion and interaction between the micromirror plate and the firstsacrificial layer and are removable with the sacrificial layer are alsoapplicable.

After depositing the first sacrificial layer, mirror plate 230 isdeposited and patterned on the first sacrificial layer. Because themicromirror is designated for reflecting incident light in the spectrumof interest (e.g. visible light spectrum), it is preferred that themicromirror plate layer comprises of one or more materials that exhibithigh reflectivity (preferably 90% or higher) to the incident light. Thethickness of the micromirror plate can be wide ranging depending uponthe desired mechanical (e.g. elastic module), the size of themicromirror, desired ON state angle and OFF state angle, and electronic(e.g. conductivity) properties of the mirror plate and the properties ofthe materials selected for forming the micromirror plate. In anembodiment of the invention, the mirror plate is a multi-layeredstructure, which comprises a SiO_(x) layer with a preferred thicknessaround 400 Å, a light reflecting layer of aluminum with a preferredthickness around 2500 Å, and a titanium layer with a preferred thicknessaround 80 Å. In addition to aluminum, other materials, such as Ti,AlSiCu, Ag, Au and TiAl, having high reflectivity to visible light canalso be used for the light reflecting layer. These mirror plate layerscan be deposited by PVD at a temperature preferably around 150° C.

After deposition, the mirror plate layer is patterned into a desiredshape, such as that in FIG. 4A. The patterning of the micromirror can beachieved using standard photoresist patterning followed by etchingusing, for example CF4, C12, or other suitable etchant depending uponthe specific material of the micromirror plate layer.

Following the patterning mirror plate 232, second sacrificial layer 242is deposited on the mirror plate 232 and first sacrificial layer 240.The second sacrificial layer may comprise amorphous silicon, or couldalternatively comprise one or more of the various materials mentionedabove in reference to the first sacrificial layer. First and secondsacrificial layers need not be the same, though are the same in thepreferred embodiment so that, in the future, the etching process forremoving these sacrificial materials can be simplified. Similar to thefirst sacrificial layer, the second sacrificial layer may be depositedusing any suitable method, such as LPCVD or PECVD. In an embodiment ofthe invention, the second sacrificial layer comprises amorphous silicondeposited around 350° C. The thickness of the second sacrificial layercan be on the order of 9000 Å, but may be adjusted to any reasonablethickness, such as between 2000 Å and 20,000 Å depending upon thedesired distance (in the direction perpendicular to the micromirrorplate and the substrate) between the micromirror plate and the hinge.

As an alternative feature of the embodiment, a second barrier layer maybe deposited between the mirror plate and the second sacrificial layerfor preventing the diffusion between the mirror plate and the secondsacrificial layer. Though the second barrier layer may be different fromthe first barrier layer, it is preferred that the two barrier layers arethe same such that the etching process afterwards for removing thesacrificial layers and the barrier layers can be simplified.

The deposited second sacrificial layer is then patterned for forming twodeep-via 218 as shown in FIG. 6 b, shallow via 228 as shown in FIG. 6 a,extension-plate post 234 and stop 230 in FIG. 6 a using standardlithography technique followed by etching. The etching step may beperformed using Cl₂, BCl₃, or other suitable etchant depending upon thespecific material(s) of the second sacrificial layer. The distanceacross the two deep-via areas depends upon the length of the defineddiagonal of the micromirror plate. In an embodiment of the invention,the distance across the two deep-via areas after the patterning ispreferably around 10 μm, but can be any suitable distance as desired. Inorder to form the shallow-via area, an etching step using CF₄ or othersuitable etchant may be executed. The shallow-via area, which can be ofany suitable size, is preferably on the order of 2.2 microns. And thesize of each deep-via is around 0.5 micron.

After patterning the second sacrificial layer, hinge support layers 244and 246 are deposited on the patterned second sacrificial layer, asshown in FIG. 6 a and FIG. 6 b. Because the hinge support layers aredesignated for holding the hinge (e.g. hinge 227 in FIG. 5 a) and themirror plate (e.g. mirror plate 232 in FIG. 5 a) attached therewith suchthat the mirror plate can rotate, it is desired that the hinge supportlayers comprise of materials having at least large elastic modulus.According to an embodiment of the invention, layer 244 comprises a 400 Åthickness of TiN_(x) (although it may comprise TiN_(x), and have athickness between 100 Å and 2000 Å) layer deposited by PVD, a 3500 Åthickness of SiN_(x) layer 246 (although the thickness of the SiN_(x)layer may be between 2000 Å and 10,000 Å) deposited by PECVD. Of course,other suitable materials and methods of deposition may be used (e.g.methods, such as LPCVD or sputtering), and a 400 Å thickness of TiN_(x).The TiN_(x) layer is not necessary for the invention, but provides aconductive contact surface between the micromirror and the hinge inorder to, at least, reduce charge-induced stiction.

After the deposition, layers 244 and 246 are patterned into a desiredconfiguration (e.g. hinge support 219 and stop 230 in FIG. 5 a). The twolayers fill the trench 234 for the extension-plate. An etching stepusing one or more proper etchants is then performed afterwards. Inparticular, the layers can be etched with a chlorine chemistry or afluorine chemistry where the etchant is a perfluorocarbon orhydrofluorocarbon (or SF₆) that is energized so as to selectively etchthe hinge support layers both chemically and physically (e.g. aplasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. or morelikely combinations of the above or with additional gases, such asCF₄/H₂, SF₆/Cl₂, or gases using more than one etching species such asCF₂Cl₂, all possibly with one or more optional inert diluents).Different etchants may, of course, be employed for etching each hingesupport layer (e.g. chlorine chemistry for a metal layer, hydrocarbon orfluorocarbon (or SF₆) plasma for silicon or silicon compound layers,etc.).

After etching the hinge support layers, two posts 218, hinge contact 228mirror-extension post 236 and stop 230 are formed. The bottom segmentsof hinge contact 228 and mirror-extension 234 are removed by etching andportions of the mirror plate underneath the hinge contact and theextension-plate are thus exposed. The exposed portions of the mirrorplate will be used to form electric-contact with external electricsource. The sidewalls of the hinge contact 216 and mirror-extension postare left with residues of layers 244 and 246 after etching. The residueon the sidewalls helps to enhance the mechanical and electricalproperties of the hinge that will be formed afterwards.

After the completion of patterning and etching of layers 244 and 246,hinge layer 248 is deposited and then patterned as shown in FIG. 6 a andFIG. 6 b. In the embodiment of the invention, the hinge layer iselectrically conductive. Examples of suitable materials for the hingelayer are Al, Ir, titanium, titanium nitride, titanium oxide(s),titanium carbide, TiSiN_(x), TaSiN_(x), or other ternary and highercompounds. When titanium is selected for the hinge layer, it can bedeposited at 100° C. Alternatively, the hinge layer may comprise ofmulti-layers, such as 100 Å TiN_(x) and 400 Å SiN_(x).

Following the deposition, the hinge layer is patterned using etching forforming the hinge (e.g. hinge 227 in FIG. 5 a), the extension plate(e.g. extension-plate 234 in FIG. 5 a), the posts (e.g. posts 218 inFIG. 5 a) and the stop (e.g. stop 230 in FIG. 5 a). Similar to the hingesupport layers (layers 236 and 238), hinge layer 242 can be etched witha chlorine chemistry or a fluorine chemistry where the etchant is aperfluorocarbon or hydrofluorocarbon (or SF₆) that is energized so as toselectively etch the hinge layers both chemically and physically (e.g. aplasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. or morelikely combinations of the above or with additional gases, such asCF₄/H₂, SF₆/Cl₂, or gases using more than one etching species such asCF₂Cl₂, all possibly with one or more optional inert diluents).Different etchants may, of course, be employed for etching each hingelayer (e.g. chlorine chemistry for a metal layer, hydrocarbon orfluorocarbon (or SF₆) plasma for silicon or silicon compound layers,etc.).

In the above example, the extension plate has the same material as thehinge, and the extension plate is spaced from the mirror plate with thesame distance as the hinge. Thereby, the extension plate can befabricated (e.g. deposited and etched) at the same time. If theextension plate comprises a material that is different from the hinge,the hinge and the extension plate can be fabricated separately on thepatterned hinge support layers. In this case, a third sacrificial layermay be deposited and a separate etching step may be performed. Forexample, the third sacrificial layer is deposited on the patterned hingelayer (the hinge material at the extension-plate 234 will be removedafter patterning the hinge layer). And the third sacrificial layer ispatterned so as to expose extension-plate 234 for depositing theextension-plate that comprises different material from the hinge. Thenthe deposited mirror-extension-plate is patterned to form the desiredextension plate. In the above example, the hinge is formed followed byforming the extension plate. Alternatively, the extension plate can beformed before forming the hinge. Similar processes (e.g. depositing andpatterning the mirror-extension layer and depositing and patterning thethird sacrificial layer followed by depositing and patterning the hingelayer on the patterned ) can be applied, which will not be discussedfurther.

Finally, the micromirror device is released by removing the sacrificiallayers using proper etching process with selected etchants. The releaseetching utilizes an etchant gas capable of spontaneous chemical etchingof the sacrificial material, preferably isotropic etching thatchemically (and not physically) removes the sacrificial material. Suchchemical etching and apparatus for performing such chemical etching aredisclosed in U.S. patent application Ser. No. 09/427,841 to Patel et al.filed Oct. 26, 1999, and in U.S. patent application Ser. No. 09/649,569to Patel at al. filed Aug. 28, 2000, the subject matter of each beingincorporated herein by reference. Preferred etchants for the releaseetch are gas phase fluoride etchants that, except for the optionalapplication of temperature, are not energized. Examples include HF gas,noble gas halides such as xenon difluoride, and interhalogens such asIF₅, BrCl₃, BrF₃, IF₇ and ClF₃. The release etch may comprise additionalgas components such as N₂ or an inert gas (Ar, Xe, He, etc.). In thisway, the remaining sacrificial material is removed and themicromechanical structure is released. In one aspect of such anembodiment, XeF₂ is provided in an etching chamber with diluents (e.g.N₂ and He). The concentration of XeF₂ is preferably 8 Torr, although theconcentration can be varied from 1 Torr to 30 Torr or higher. Thisnon-plasma etch is employed for preferably 900 seconds, although thetime can vary from 60 to 5000 seconds, depending on temperature, etchantconcentration, pressure, quantity of sacrificial material to be removed,or other factors. The etch rate may be held constant at 18 Å/s/Torr,although the etch rate may vary from 1 Å/s/Torr to 100 Å/s/Torr. Eachstep of the release process can be performed at room temperature.

In addition to the above etchants and etching methods mentioned for usein either the final release or in an intermediate etching step, thereare others that may also be used by themselves or in combination. Someof these include wet etches, such as ACT, KOH, TMAH, HF (liquid); oxygenplasma, SCCO₂, or super critical CO₂ (the use of super critical CO₂ asan etchant is described in U.S. patent application Ser. No. 10/167,272,which is incorporated herein by reference). Of course, the etchants andmethods selected should be matched to the sacrificial materials beingremoved and the desired materials being left behind.

Though the exemplary fabrication process of the embodiment of thepresent invention is discussed with reference to FIG. 6 a and FIG. 6 b,wherein only one micromirror is illustrated, the fabrication process,however, is executed for the entire micromirror array containingmillions of micromirrors, such as the micromirror in FIG. 4A. Afterreleasing the micromirror array by etching the sacrificial layers andbarrier layers (if presented), the micromirror array is preferablypackaged for protection purpose. Exemplary packaging methods andmaterials used in packaging are disclosed in Ser. No. 10,366,296 toPatel, filed Feb. 12, 2003, the subject of which being incorporatedherein by reference. The packaged micromirror array can thus be employedin display systems, such as those in FIG. 1 and FIG. 2. In operation,the micromirrors of the micromirror array are individually selectable,and the selected micromirror is driven to rotate to either an ON stateor an OFF state (including the non-deflected state or the state parallelto the substrate). The selection and the actuation are achieved byelectrostatic fields established between the mirror plates of themicromirrors and the electrodes associated with the mirror plates. Theelectrostatic fields are controlled by an array of electrical circuitry,such as an array of DRAMs or an array of “charge pump pixel cells” asset forth in U.S. patent application Ser. No. 10,407,061 to Richards,filed Apr. 2, 2003, the subject of which being incorporated herein byreference. Under the control of the circuitry array, the packagedmicromirror array in a display system modulates light from a lightsource of the display system for producing images on a display target.U.S. patent applications Ser. No. 10,607,687 to Richards, filed Jun. 27,2003 contain examples of operation methods that can be used to operatethe micromirror and the micromirror array, especially in displayapplications, the subject of which being incorporated herein byreference.

It will be appreciated by those of skill in the art that a new anduseful spatial light modulator containing a micromirror array and amethod of fabricating the spatial light modulator have been describedherein. In view of many possible embodiments to which the principles ofthis invention may be applied, however, it should be recognized that theembodiments described herein with respect to the drawing figures aremeant to be illustrative only and should not be taken as limiting thescope of invention. For example, those of skill in the art willrecognize that the illustrated embodiments can be modified inarrangement and detail without departing from the spirit of theinvention. In particular, the micromirrors and the electrode andcircuitry can be formed on the same substrate. The sacrificial layer andetchant disclosed herein are exemplary only. Other suitable sacrificialmaterials, such as silicon dioxide could also be used and removed withHF (or HF/HCl). Alternatively, a silicon sacrificial material can beused in fabricating the micromirror and removed with ClF₃ or BrF₃. A PSGcould also be used as sacrificial material and removed with buffered HF,or an organic sacrificial such as polyimide could be removed in a dryplasma oxygen release step. Of course the etchant and sacrificialmaterial should be selected depending upon the structural material to beused. Also, though PVD and CVD are referred to above, other thin filmdeposition methods could be used for depositing the layers, includingspin-on, sputtering, anodization, oxidation, electroplating andevaporation. Therefore, the invention as described herein contemplatesall such embodiments as may come within the scope of the followingclaims and equivalents thereof.

1. A projector, comprising: a rectangular array of reflective anddeflectable mirror plates on a substrate; a light source providing lightonto the rectangular array of mirror plates; wherein the light has afirst angle from 50° to 70° degrees to the substrate; wherein the lighthas a second angle to a side of the rectangular array when viewed fromthe top of the mirror plates, said second angle being from 50° to 65°degrees; and projection optics for projecting the light reflected fromthe mirror plates onto a screen.
 2. The projector of claim 1, whereinthe substrate is a light transmissive substrate.
 3. The projector ofclaim 1, wherein the substrate is a semiconductor substrate.
 4. Theprojector of claim 1, wherein the substrate has four predominant sidesthat form a rectangular shape; and wherein each side of the mirror plateis at an angle of from 5° degrees to 25° degrees to the two sides of therectangular substrate.
 5. The projector of claim 1, wherein each mirrorplate is in a shape of a rectangle, square, trapezoid or rhombus.
 6. Theprojector of claim 1, wherein the mirror plate is in a shape of square.7. The projector of claim 1, wherein at least one of the mirror platesis supported by two posts formed on the substrate.
 8. The projector ofclaim 1, wherein the substrate has four predominant sides that form arectangular shape; and wherein each side of the mirror plate is at anangle of from 10° degrees to 20° degrees to the two sides of therectangular substrate.
 9. The projector of claim 1, wherein the mirrorplate is attached to the hinge such that the mirror plate is capable ofrotating along a rotation axis that is parallel to but offset from adiagonal of the mirror plate when viewed from the top of the substrate.10. The projector of claim 1, wherein the micromirror further comprises:a first electrode placed proximate to the mirror plate such that a firstelectrical filed is established between the first electrode and themirror plate, and the mirror plate rotates relative to the substrate ina first rotation direction in response to the first electrical field.11. The projector of claim 10, wherein the micromirror furthercomprises: a second electrode placed proximate to the mirror plate suchthat a second electrical filed is established between the secondelectrode and the mirror plate, and the mirror plate rotates relative tothe substrate in a second rotation direction in response to the firstelectrical field, wherein the second rotation direction is opposite tothe first rotation direction.
 12. The projector of claim 11, wherein thefirst and the second electrode are on a substrate other than thesubstrate to which the hinge support is connected.
 13. The projector ofclaim 11, wherein the first electrode is on a substrate other than thesubstrate to which the hinge support is connected; and wherein thesecond electrode is on the substrate to which the hinge support isconnected.
 14. The projector of claim 11, wherein the second electrodeis an electrode film on a surface of the substrate to which the hingesupport is connected.
 15. The projector of claim 1, wherein themicromirror further comprises: an extension plate connected to themirror plate.
 16. The projector of claim 15, wherein the extension plateis connected to the mirror plate via an extension-plate post and theextension plate defines a first gap between the extension and the mirrorplate.
 17. The projector of claim 15, wherein the extension plate isextended beyond the mirror plate and connected to the mirror plate viaan extension-plate post; and wherein the extension plate defines asecond gap between the extension plate and the substrate to which thehinge support is connected.
 18. The projector of claim 15, wherein theextension plate is electrically conducting.
 19. The projector of claim15, wherein the extension plate is dielectric with a dielectric constantlarger than 1.0.
 20. The projector of claim 1, wherein the micromirrorfurther comprises: a first stop for stopping the rotation of the mirrorplate when the mirror plate rotates to an ON state angle.
 21. Theprojector of claim 20, wherein the first stop is disposed on the hingesupport.
 22. The projector of claim 20, wherein the micromirror furthercomprises: a second stop for stopping the rotation of the mirror platewhen the mirror plate rotates to an OFF state.
 23. The projector ofclaim 1, further comprising: collection optics for directing the lightfrom the light source onto the mirror plates.